U.S. patent application number 15/104143 was filed with the patent office on 2016-11-03 for device and method for melting a material without a crucible and for atomizing the melted material in order to produce powder.
The applicant listed for this patent is NANOVAL GMBH & CO. KG. Invention is credited to Christian Gerking, Luder Gerking, Rico Heinz, Martin Stobik.
Application Number | 20160318105 15/104143 |
Document ID | / |
Family ID | 52278634 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160318105 |
Kind Code |
A1 |
Gerking; Luder ; et
al. |
November 3, 2016 |
DEVICE AND METHOD FOR MELTING A MATERIAL WITHOUT A CRUCIBLE AND FOR
ATOMIZING THE MELTED MATERIAL IN ORDER TO PRODUCE POWDER
Abstract
The invention relates to a device (1) for melting a material
without a crucible and for atomizing the melted material in order
to produce powder, comprising: an atomizing nozzle (5); an
induction coil (4) having windings (4a-d), which become narrower in
the direction of the atomizing nozzle (5) at least in some
sections; and a material bar (3) at least partially inserted into
the induction coil (4). The induction coil (4) is designed to melt
the material of the material bar (3) in order to produce a melt
flow (16). The induction coil (4) and the atomizing nozzle (5) are
arranged in such a way that the melt flow (16) is or can be
introduced into the atomizing nozzle (5) through a first opening
(20) of the atomizing nozzle (5) in order to atomize the melt flow
(16) by means of an atomizing gas (19), which can be introduced
into the atomizing nozzle (5). The device is characterized in that
the atomizing nozzle (5) is designed in such a way that the
atomizing gas (19) can be or is introduced into the atomizing
nozzle (5) only through said first opening (20) of the atomizing
nozzle (5); that the atomizing nozzle (5) is designed to accelerate
the atomizing gas (19) at least up to the speed of sound of the
atomizing gas (19) in a direction parallel to the melt flow (16);
and that the material bar (3) and the induction coil (4) are
arranged in such a way that the melt flow (16) can be or is
inductively heated by the induction coil (4) before the melt flow
(16) enters the atomizing nozzle (5). The invention further relates
to a corresponding method for melting a material without a crucible
and for atomizing the melted material in order to produce
powder.
Inventors: |
Gerking; Luder; (Berlin,
DE) ; Gerking; Christian; (Forst, DE) ;
Stobik; Martin; (Berlin, DE) ; Heinz; Rico;
(Liebenwalde, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NANOVAL GMBH & CO. KG |
Berlin |
|
DE |
|
|
Family ID: |
52278634 |
Appl. No.: |
15/104143 |
Filed: |
December 19, 2014 |
PCT Filed: |
December 19, 2014 |
PCT NO: |
PCT/EP2014/078849 |
371 Date: |
June 13, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2009/0896 20130101;
B22F 2009/0848 20130101; B22F 2009/0892 20130101; B22F 2009/0824
20130101; B22F 2202/07 20130101; C04B 35/626 20130101; B22F 1/0014
20130101; B22F 2304/10 20130101; B22F 2009/0836 20130101; H05B
6/101 20130101; B22F 2999/00 20130101; C04B 2235/786 20130101; B22F
9/082 20130101; B22F 2998/10 20130101; B22F 2999/00 20130101; B22F
9/082 20130101; B22F 3/003 20130101; B22F 2999/00 20130101; B22F
2009/0892 20130101; B22F 2202/07 20130101 |
International
Class: |
B22F 9/08 20060101
B22F009/08; H05B 6/10 20060101 H05B006/10; C04B 35/626 20060101
C04B035/626; B22F 1/00 20060101 B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2013 |
DE |
10 2013 022 096.3 |
Claims
1. A device for the crucible-free melting of a material and for
atomising the melted material for manufacturing powder, the device
comprising an atomisation nozzle; an induction coil with windings,
which at least in sections become narrower in the direction towards
the atomisation nozzle; and a material rod which is introduced at
least partly into the induction coil; wherein the induction coil is
configured to melt the material of the material rod for producing a
melt flow; and wherein the induction coil and the atomisation
nozzle are arranged in a manner such that the melt flow can be
introduced or is introduced into the atomisation nozzle through a
first opening of the atomisation nozzle, for the atomisation of the
melt flow by way of an atomisation gas which can be introduced into
the atomisation nozzle, and wherein the atomisation nozzle is
configured such that the atomisation gas can only be introduced or
is only introduced into the atomisation nozzle through the
mentioned first opening of the atomisation nozzle; that the
atomisation nozzle is configured to accelerate the atomisation gas
in a direction parallel to the melt flow, at least up to the speed
of sound of the atomisation gas; and that the material rod and the
induction coil are arranged in a manner such that the melt flow is
inductively heatable or heated by the induction coil, before entry
of the melt flow into the atomisation nozzle.
2. The device according to claim 1, wherein the material, from
which the material rod is formed, comprises metal or ceramic,
wherein the metal contains aluminium, iron or titanium.
3. The device according to claim 1, wherein a smallest inner
diameter d.sub.min of the atomisation nozzle is smaller than 7
mm.
4. The device according to claim 1, wherein a plane which is given
by a minimal cross-sectional area of the atomisation nozzle which
is determined perpendicularly to the nozzle axis of the atomisation
nozzle or perpendicularly to the flow direction of the melt flow,
wherein the material rod and the atomisation nozzle are arranged in
a manner such that for a smallest distance L between the material
rod and the mentioned plane, the following applies:
L.ltoreq.5d.sub.min
5. The device according to claim 1, wherein the atomisation nozzle
and the induction coil are separate components.
6. The device according to claim 1, wherein a smallest distance
d.sub.min between the induction coil and the plane given by the
minimal cross-sectional area of the atomisation nozzle which is
determined perpendicularly to the nozzle axis of the atomisation
nozzle, the following applies: a.sub.min.ltoreq.4d.sub.min, wherein
the distance a.sub.min is determined in a direction parallel to the
nozzle axis or parallel to the flow direction of the melt flow.
7. The device according to claim 1, wherein the atomisation nozzle,
for the minimisation of a heat quantity dissipated by the activity
of the induction coil in the atomisation nozzle, is formed from a
nozzle material, for whose specific electrical resistance .rho.,
the following applies: .rho..ltoreq.0.0210.sup.-6 .OMEGA.m or
.rho..gtoreq.10.sup.-2 .OMEGA.m.
8. The device according to claim 1, wherein a high-pressure
chamber, an atomisation chamber which is in fluid connection with
the high-pressure chamber via the atomisation nozzle, first
pressure controller for introducing the atomisation gas into the
high-pressure chamber and for the control of a first gas pressure
p.sub.1 in the high-pressure chamber, as well as second pressure
controller for the control of a second gas pressure p.sub.2 in the
atomisation chamber, wherein the first and the second pressure
controllers are configured to set the pressures p.sub.1 and p.sub.2
for accelerating the atomisation gas in a direction parallel to the
flow direction of the melt flow, in a manner such that:
p.sub.1/p.sub.2>1.8 and p.sub.1>10 bar.
9. The device according to claim 1, wherein at least one of the
windings of the induction coil which is arranged in the region of
the end of the material rod which faces the atomisation nozzle, and
specifically at least the last winding of the induction coil which
faces the atomisation nozzle, at least in sections runs
perpendicularly to the rod axis, for producing an electromagnetic
field which with respect to the rod axis of the material rod is
symmetrical in this region.
10. The device according to claim 9, wherein at least one winding
is a ring conductor which is electrically interrupted at a
location.
11. The device according to claim 10, wherein at least two of these
ring conductors, wherein the at least two ring conductors are
connected electrically in parallel.
12. The device according to claim 11, Wherein ring conductors with
a different periphery have different cross sections and/or have
different distances to one another along the coil axis, so that
they each have an approximately equal electrical resistance and
produce a homogeneous field distribution along the rod axis.
13. The device according to claim 1, wherein the induction coil at
least in sections is wound in a spiral manner, and in a continuous
manner with a pitch which is different to zero with respect to a
direction parallel to the rod axis, wherein the windings in this
section run on the envelope of a cone symmetrical to the rod
axis.
14. The device according to claim 1, wherein a conductor forming
the induction coil is a hollow tube for leading a cooling
fluid.
15. The device according to claim 1, wherein at least one further
nozzle which is arranged aligned to the atomisation nozzle and
which is arranged between the material rod and the atomisation
nozzle, so that the melt flow can also be led or is led through the
further nozzle, wherein the further nozzle is configured to
accelerate a gas introduced with the melt flow into the further
nozzle, in a direction parallel to the melt flow, at least up to
0.5-fold the speed of sound of the gas introduced into the further
nozzle.
16. A method for manufacturing powder by way of crucible-free
melting of a material and by way of atomisation of the melted
material, the method comprising the steps of: at least partly
introducing a material rod into an induction coil which tapers
conically at least in sections; subjecting the induction coil to an
alternating voltage for melting the material rod and for producing
a melt flow; introducing the melt flow into an atomisation nozzle
through a first opening of the atomisation nozzle; and introducing
an atomisation gas into the atomisation nozzle and atomising the
melt flow by way of the atomisation gas; wherein the atomisation
gas is only introduced into the atomisation nozzle through the
first opening of the atomisation nozzle; wherein the atomisation
gas which is to be introduced and/or is introduced into the
atomisation nozzle through the first opening is accelerated in a
direction parallel to a flow direction of the melt flow, at least
up to the speed of the sound of the atomisation gas, so that the
melt flow divides up or even bursts and powder with a grain size in
the micrometer range and/or sub-micrometer range is produced; and
wherein the melt flow is inductively heated by the induction coil
before the entry of the melt flow into the atomisation nozzle.
17. The method according to claim 16, wherein the atomisation gas
is accelerated parallel to the flow direction of the melt flow
along an acceleration path with a length L.sub.B, at least to the
speed of sound of the atomisation gas, wherein for L.sub.B the
following applies: L.sub.B.ltoreq.5d.sub.min, wherein d.sub.min is
the smallest diameter of the atomisation nozzle perpendicular to
the nozzle axis.
18. The method according to claim 16, wherein the melt flow is led
through a further nozzle which is arranged aligned to the
atomisation nozzle, and wherein a gas introduced with the melt flow
into the further nozzle is accelerated in a direction parallel to
the melt flow, at least up to 0.5-fold the speed of sound of the
gas introduced into the further nozzle.
19. The powder, in particular a metal powder or ceramic powder,
manufactured by the method according to claim 16.
20. The powder according to claim 19, wherein a mass-related mean
grain diameter which is smaller than 50 .mu.m.
21. The powder according to claim 19 wherein 84 percent of the
powder (percentage by weight) has a grain diameter which is smaller
than d.sub.84 and that 50% of the powder (percentage by weight) has
a grain diameter which is smaller than d.sub.50, wherein the
following applies: d.sub.84/d.sub.50.ltoreq.2.8.
22. The powder according to claim 19 wherein the powder is
manufactured from one of the following metals or of an alloy of one
or more of the following metals: titanium, aluminium, iron,
zirconium, hafnium, vanadium, niobium, tantalum, chromium,
molybdenum, wolfram, rhenium, nickel, cobalt.
Description
[0001] The invention relates to a method and to a device for the
crucible-free melting of a material and for atomising the melted
material, for manufacturing power, in particular for manufacturing
metal or ceramic powder.
[0002] Metal powder is applied in many fields of technology. Metal
powder is produced with the powder injection moulding method (PIM)
or also with generative methods, also called additive methods, such
as laser sintering/melting and electron beam melting, and can often
be melted into complex three-dimensional structures. Metal powders
with grain sizes in the micrometer region are often required.
Thereby, for many applications, it is extremely important that the
grain size of the metal powder does not exceed a maximal grain size
and that a fluctuation width of a statistical grain size
distribution of the manufactured powder is as small as possible,
thus that the grain size deviates as little as possible from a
desired grain size.
[0003] From the patent document DE10340606B4, it is known to melt
metal in a crucible and to atomise it into metal powder by way of a
Laval nozzle. Thereby, it is extremely important to thermally
shield the melt nipple, with which the metal melted in the crucible
is introduced into the nozzle, with respect to the cold atomisation
gas, since the melt otherwise cools to greatly, which significantly
worsens the quality of the produced powder (grains shape, grain
size, grain size distribution width) or renders the atomisation
impossible. A suitable shielding has therefore been suggested in
DE10340606B4. One challenge was to design the shielding in a manner
that it does not disadvantageously influence the flow profile of
the atomisation gas before entry into the nozzle, since this flow
profile also has a considerable influence on the quality of the
produced powder.
[0004] A disadvantage of the device described in DE10340606B4 lies
in the fact that no materials which chemically react with the
crucible coating and become impure on account of this reaction can
be melted and pulverised in the crucible. This problem occurs for
example with the crucible melting of titanium. A device for the
crucible-free melting of metal has therefore already been suggested
in DE4102101A1. Thereby, a metal rod is melted by way of an
induction coil and subsequently likewise atomised by way of an
atomisation nozzle. However, the problem of the melt flow which is
produced on melting the rod being greatly cooled by the atomisation
gas occurs to an even more significant extent with the device
according to DE4102101A1 than with the device according to
DE10340606B4.
[0005] A completely different type of atomisation is suggested with
the device according to DE4102101A1, in order to circumvent this
problem. According to this, the nozzle comprises a first opening,
through which the melt flow is introduced into the nozzle. The
atomisation gas again is led into the nozzle through a lateral
opening of the nozzle which is different to the first opening, thus
in a direction perpendicular to the flow direction of the melt flow
through the nozzle. The atomisation gas in the nozzle hits the melt
flow perpendicularly with a great impulse and breaks up the melt
flow, so that drops are formed, which subsequently freeze into
powder. Essentially the same type of atomisation is also described
in EP1765536B1. The cooling of the melt flow before entering into
the nozzle is at least partly prevented by way of the lateral
introduction of the atomisation gas into the nozzle.
[0006] However, it has been found that only powder of a
comparatively large grain size distribution width can be
manufactured with the type of atomisation which has been put
forward in DE4102101A1 and EP1765536B1. A desired grain size thus
under certain circumstances can only be set to an inadequate
precision, so that much wastage occurs as the case may be. The
manufacturing costs can increase due to this.
[0007] It is therefore the object of the present invention, to
create a device and a method, with which an as large as possible
multitude of materials can be pulverised, wherein a grain size and
the grain size distribution of the manufactured powder can be set
as precisely as possible.
[0008] This object is achieved by a device and a method according
to the independent claims. Special embodiments are described in the
dependent claims.
[0009] What is put forward therefore is a device for the
crucible-free melting of a material and for atomising the melted
material for manufacturing powder, comprising [0010] an atomisation
nozzle; [0011] an induction coil with windings, which at least in
sections become narrower in the direction towards the induction
coil; and [0012] a material rod which is introduced at least partly
into the induction coil; [0013] wherein the induction coil is
configured to melt the material of the material rod for producing a
melt flow; and [0014] wherein the induction coil and the
atomisation nozzle are arranged in a manner such that the melt flow
can be introduced into the atomisation nozzle through a first
opening of the atomisation nozzle, for atomisation the melt flow by
way of an atomisation gas which can be introduced into the
atomisation nozzle.
[0015] The atomisation nozzle is designed in a manner such that the
atomisation gas can only be introduced or is only introduced into
the atomisation nozzle through the mentioned first opening of the
atomisation nozzle. Moreover, the atomisation nozzle is configured
to accelerate the atomisation gas in a direction parallel to the
melt flow, preferably parallel to a flow direction of the melt flow
through the atomisation nozzle, at least up to the speed of sound
of the atomisation gas. The material rod and the induction coil are
moreover arranged in a manner such that the melt flow is
inductively heatable or heated by the induction coil, before entry
of the melt flow into the atomisation nozzle, thus typically in a
region between an end of the material rod which faces the
atomisation nozzle, and the atomisation nozzle The induction coil,
in particular at its end which faces the atomisation nozzle, is
designed in a manner such that the melt flow, where it freely
flows, is inductively heatable or heated, such that it does not
cool down. It is heated for example in a manner such that it
retains its minimum temperature necessary for the atomisation
process. The melt flow is mostly subjected to the atomisation gas
in the region, in which the melt flow freely flows, thus normally
between the end of the material rod which faces the atomisation
nozzle, and the atomisation nozzle, said atomisation gas enclosing
and flowing around the melt flow there at all sides.
[0016] What is also suggested in a method for manufacturing powder
by way of crucible-free melting of a material and by way of
atomisation of the melted material, comprising the steps: [0017] at
least partly introducing a material rod into an induction coil
which tapers conically at least in sections; [0018] subjecting the
induction coil to an alternating voltage for melting the material
rod and for producing a melt flow; [0019] introducing the melt flow
into an atomisation nozzle through a first opening of the
atomisation nozzle; and [0020] introducing an atomisation gas into
the atomisation nozzle and atomising the melt flow by way of the
atomisation gas; [0021] wherein the atomisation gas is only
introduced into the atomisation nozzle through the first opening of
the atomisation nozzle; [0022] wherein the atomisation gas which is
to be introduced and/or is introduced into the atomisation nozzle
through the first opening is accelerated in a direction parallel to
a flow direction of the melt flow, preferably parallel to a flow
direction of the melt flow through the atomisation nozzle, at least
up to the speed of the sound of the atomisation gas, so that the
melt flow is divided up or even bursts and powder with a grain size
in the micrometer range and/or sub-micrometer range is produced;
and [0023] wherein the melt jet is inductively heated by the
induction coil before the entry of the melt jet into the
atomisation nozzle.
[0024] The atomisation nozzle, the induction coil and the material
rod, for the sake of simplicity are hereinafter also called nozzle,
coil and rod. The complete or essentially the complete atomisation
gas which is envisaged for atomising the melt is introduced into
the nozzle through the same first opening of the nozzle as the melt
flow. The first opening of the nozzle usually faces the coil and
the rod. Then it is normally the atomisation gas and the completely
or partly pulverised melt flow which exits again out of the nozzle
through a second opening of the nozzle. The nozzle thus apart from
the first and the second opening preferably comprises no further
openings, in particular no lateral openings, for introducing gas
perpendicularly or essentially perpendicular to the nozzle axis, as
is the case e.g. with the devices according to DE4201101A1 and
EP1765536B1.
[0025] It has been found that a multitude of different materials
can be atomised in a crucible-free manner for manufacturing powder
with very good results with the device suggested here and with the
method suggested here. Thus powder with a narrow grain diameter
distribution width can be manufactured, wherein the desired grain
diameter and distribution can be well set by a number of process
parameters and/or device parameters. A significant advantage lies
in the fact that materials Which cannot be melted in a crucible due
to the fact that the material which is to be nozzle atomised at
very high temperatures melts the crucible material or reacts with
this and thus becomes impure, can also be atomised. A cooling or
freezing of the melt flow before the atomisation is effectively
prevented by way of the heating of the melt flow by way of the
coil.
[0026] Normally, the material rod, the coil and the atomisation
nozzle are aligned along a vertical direction, along which the
gravity acts. The melt flow then falls through the atomisation
nozzle under the influence of gravity or at least also under the
influence of gravity. The rod, the coil and the nozzle can each
have a cylindrical symmetry or approximately a cylindrical
symmetry, wherein the rod, the coil and the nozzle are then
typically arranged in a manner such that their axes of symmetry are
arranged on the same straight line. The rod, the coil and the
nozzle however can also basically have arbitrarily shaped cross
sections. The nozzle for example can have a slot-like, rectangular,
oval or round cross section. The rod can likewise have a round,
oval or polygonal cross section. The rod can also be designed in a
plate-shaped manner. The shapes of the coil and the nozzle are then
to be accordingly adapted to the plate shape of the rod.
[0027] The coil usually comprises at least three windings,
preferably between three and six windings. The dimensions of the
coil and the rod are preferably adapted to one another, so that a
more efficient energy transfer from the coil onto the rod can be
effected for melting the rod. The coil is preferably subjected to
an alternating voltage f, which lies roughly between 50 kHz and 200
kHz, preferably between 100 kHz and 150 kHz. The coil is usually
operated with a power between 10 and 150 kW for melting the rod,
depending of the material of the rod.
[0028] A cross-sectional area of the nozzle along the nozzle axis
in the flow direction of the melt flow through the nozzle can
reduce in a continuous manner or at least in sections. The
cross-sectional area of the nozzle along the nozzle axis in the
flow direction for example can reduce linearly or more greatly than
linearly. The nozzle can e.g. be designed as a Laval nozzle.
[0029] The Laval nozzle can then have a contour which runs out
radially far from the axis of the Laval nozzle, so that the flow
from the condition of calm environment to the accelerated gas is
led through the Laval nozzle already at a large distance to the
axis of the Laval nozzle. A diameter of the contour of the Laval
nozzle e.g. in the region of the first opening of the nozzle can be
roughly 0.5 times to threefold the coil diameter, preferably 0.8 to
double. With regard to the mentioned coil diameter, it can be the
case of the coil diameter at the end of the coil which is away from
the nozzle or the end of the coil which faces the nozzle.
[0030] One variant is the co-called two-stage nozzle, with which
two differently curved nozzle contours merge into one another such
that an annular edge arises in a plane perpendicular to the nozzle
axis.
[0031] The device typically comprises a lifting and lowering device
for holding, lifting and lowering the material rod. The rod e.g. is
continuously fed into the coil, in order to hold the position of
the end of the rod which faces the nozzle and at which the rod is
predominately melted, approximately constant during the
implementation of the method. The lifting and lowering device is
preferably additionally configured to rotate the rod about the rod
axis, e.g. at a rotation speed of at least 1 min.sup.-1, so that
the melting of the rod is effected as uniformly as possible.
[0032] If hereinafter, one speaks of the material rod, the material
rod is to be understood in that it is solid and not yet melted. The
melt or the melt flow, in particular with regard to the distance
measurements between the rod and other components of the device are
not to be considered as part of the material rod.
[0033] The material, from which the rod is formed, can comprise
metal or ceramic. The material of the material rod, from which the
powder is manufactured, can e.g. contain one of the following
metals or an alloy of one or more of the following metals:
titanium, aluminium, iron, zirconium, hafnium, vanadium, niobium,
tantalum, chromium, molybdenum, wolfram, rhenium, nickel,
cobalt.
[0034] Powder with a mass-related mean grain diameter of less than
50 .mu.m can be manufactured with the suggested method. The mean
mass-related grain diameter can also be less than 10 .mu.m or less
than 1 .mu.m.
[0035] A width of the grain size distribution of the manufactured
powder can be characterised by the diameter d.sub.84 and d.sub.50.
These are defined as follows: [0036] 84 percent of the powder
(percentage by weight) has a grain diameter which is smaller than
d.sub.84 and 50% of the powder (percentage by weight) has a grain
diameter which is smaller than d.sub.50. [0037] E.g. powder with
which: d.sub.84/d.sub.50.ltoreq.2.8, preferably
d.sub.84/d.sub.50.ltoreq.2.3, in particular
d.sub.84/d.sub.50.ltoreq.1.8 can be manufactured with the suggested
method.
[0038] A smallest inner diameter d.sub.min of the atomisation
nozzle can be smaller than 7 mm, preferably smaller than 5 mm,
particularly preferably smaller than 3 mm for achieving a
particularly narrow grain size distribution. The inner diameter is
thereby to be determined in each case perpendicularly to the nozzle
axis or perpendicularly to the flow direction of the melt flow
through the nozzle. Typically, the smallest diameter occurs at the
position along the nozzle axis, at which the nozzle or the nozzle
tube has the smallest cross-sectional area. The inner diameter is
preferably determined along a straight line which runs through the
middle point of the cross section.
[0039] A plane, which perpendicularly intersects the nozzle axis or
the flow direction of the melt flow through the nozzle, and
specifically at that position along the nozzle axis or along the
flow direction, at which the cross-sectional area of the
atomisation nozzle, in particular thus the cross-sectional area of
the channel formed by the nozzle, is minimal, can serve for the
characterisation of the device. This plane hereinafter also is
indicated as the plane of the narrowest cross section for the sake
of simplicity.
[0040] Constructional measures can be undertaken, in order to feed
the melt to the nozzle in an adequately close manner, so that it
can be captured as much as possible by the gas flow of the
atomisation gas, in the described manner. A preferred maximal
distance of the melting region to the nozzle results due to the
necessary proximity of the coil to the material rod, indeed in the
lowermost region wherein the melting is to take place in a complete
manner, so that no remains of the rod remain &symmetrically or
out-of-centre, thus Where a cooling and freezing of the melt flow
on the way to the nozzle due to the cooling by the gas flow which
is mostly cold for energetic reasons and captures and accelerates
the melt flow, should be avoided at all costs. A pressure increase
in the inside of the melt jet due to shear stresses between the
quicker gas flow and it, is preferably effected on this path to the
narrowest cross section of the nozzle, whereas with other methods,
a cold gas is in contact with the melt only Dora relatively short
time, since it has no common direction with this, neither before
nor after this contact--at the most stochastically, thus in random
parts, and moreover neither needs a long contact time since the gas
already has the high kinetic energy and therefore no longer, common
paths with a possible and undesired cooling are covered.
[0041] The material rod and the atomisation nozzle for this reason
can therefore be arranged in a manner such that a smallest distance
L between the material rod and the plane of the narrowest cross
section is smaller than 7d.sub.min, smaller that 6d.sub.min,
smaller than 5d.sub.min, or smaller than 4d.sub.min. One can
therefore counteract too high a cooling of the melt flow before
atomisation for example. The rod at the end of the rod which faces
the nozzle typically roughly has a cone shape, L is then usually
the distance of the cone tip to the plane of the narrowest cross
section.
[0042] The atomisation nozzle and the induction coil are preferably
designed as separate components. The induction coil in particular
is therefore not then integrated into the nozzle. In the flow
direction, the coil with this embodiment is normally arranged in
front of or above the nozzle. The device is particularly flexible
with this. The coil for example can be easily exchanged or adjusted
relative to the nozzle. Moreover, one can more effectively prevent
the nozzle being heated too much or even melted by the coil.
[0043] The induction coil and the atomisation nozzle can be
arranged to one another such that a smallest distance a.sub.min
between the induction coil and the plane of the narrowest cross
section is smaller than 4d.sub.min, smaller than 3d.sub.min or
smaller than 2d.sub.min, in order to avoid too great a cooling or
even a freezing of the melt flow before the atomisation, and to be
able to assist the flow of the melt flow. Preferably, the distance
a.sub.min is thereby determined in a direction parallel to the
nozzle axis or parallel to the flow direction of the melt flow. The
coil can also reach directly up to the nozzle or be in contact with
the nozzle. The nozzle in this case above all should be of a
non-conductive material
[0044] The nozzle can be of a material which is either a very good
or very poor electrical conductor, in order to prevent an
electromagnetic coupling of the nozzle to the coil and to avoid an
inductive heating of the nozzle by the cod as much as possible.
Ohmic loses which are dissipated in the nozzle in the form of heat
hardly occur in the case that the nozzle material is a very good
conductor. If in contrast the nozzle material is a very poor
conductor, then no or hardly any eddy currents are induced in the
nozzle, which likewise leads to no or hardly any ohmic losses. For
this reason .rho..ltoreq.0.0210.sup.-6 .OMEGA.m or
.rho..ltoreq.10.sup.-2 .OMEGA.m for the specific electrical
resistance of the nozzle material.
[0045] A further special embodiment of the device is characterised
by a high-pressure chamber, an atomisation Chamber which is in
fluid connection with the high-pressure chamber via the atomisation
nozzle, first pressure control means for introducing the
atomisation gas into the high-pressure chamber and for the
[closed-loop] control of a first gas pressure p.sub.1 in the
high-pressure chamber, as well as second pressure control means for
the control of a second gas pressure p.sub.2 in the atomisation
chamber, wherein the first and the second pressure control means
are configured to set the pressures p.sub.1 and p.sub.2 for
accelerating the atomisation gas in a direction parallel to the
flow direction of the melt flow, in a manner such that:
p.sub.1/p.sub.2>1.8 and p.sub.1>10 bar. The first gas
pressure p.sub.1 is thus greater than the second gas pressure
p.sub.2.
[0046] The second gas pressure p.sub.2 in the atomisation chamber
is typically about 1 bar. The acceleration of the atomisation gas
in front of the nozzle, in the inside of the nozzle and behind the
nozzle can be influenced and controlled by way of the setting of
the gas pressures p.sub.1 and p.sub.2. Thus the shear stresses
which are transmitted by the atomisation gas onto the melt flow,
and, as the case may be, the degree of centring and stretching of
the melt flow can also be influenced, in particular already before
the entry of the melt flow into the nozzle and/or before the
atomisation of the melt flow. The first and second pressure control
means can e.g. each comprise one or more pumps, conduits, nozzles,
valves, a compressor and/or a high-pressure gas tank.
[0047] It is particularly advantageous to heat the material rod in
an as effective and as uniform as possible manner, above all at its
end facing the nozzle, thus typically at the lower end, since it is
here that the melting of the rod takes pace. The end of the
material rod facing the nozzle should also be arranged within the
coil, thus in the flow direction preferably should not project
beyond the end of the coil which faces the nozzle.
[0048] Moreover, is advantageous if at least one of the windings of
the induction coil which is arranged in the region of the end of
the material rod which faces the nozzle, runs perpendicularly to
the rod axis at least in sections, for producing an electromagnetic
field Which with respect to the rod axis is as symmetrical as
possible in this region. At least the last winding of the coil
which faces the nozzle, at least in sections, preferably runs
perpendicularly to the rod axis or to the flow direction of the
melt flow. At least the last winding can preferably completely or
almost completely enclose the rod axis or an imagined extension of
the rod axis in a plane perpendicular or almost perpendicular to
the rod axis.
[0049] Thus at least one of the windings, in particular the last
winding can be designed as a ring conductor which is electrically
interrupted at one location, preferably as an almost closed ring
conductor. The interruption can be realised as an air gap or by way
of an insulating material. The term ring conductor should not only
include conductors in the shape of a ring, but rather it is to
include a multitude of shapes. What is decisive is the fact that
the ring conductor over the greatest part of its length runs
essentially in a plane and forms an almost closed conductor loop.
For example, the ring conductor can run at least 50 percent, at
least 70 percent or at least 90 percent in a plane perpendicular or
essentially perpendicular to the rod axis or perpendicularly or
essentially perpendicularly to the flow direction of the melt flow.
The ring conductor can enclose the rod axis or an imagined
extension of the rod axis by at least 180 degrees, by at least 225
degrees, by at least 270 degrees, by at least 315 degrees or by
almost 360 degrees perpendicularly or almost perpendicularly to the
rod axis. The ring conductor can be designed in a predominantly
circular, oval, rectangular or polygonal manner. It can have
roughly the shape of a horseshoe. The ring conductor does not need
to have a symmetrical shape. The ring conductor however is
preferably arranged rotationally symmetrically or almost
rotationally symmetrically to the rod axis or the flow direction of
the melt flow. The ring conductor can thus e.g. be designed in a
roughly circular manner.
[0050] With a special embodiment, the coil can comprises at least
two ring conductors of the described type. The at least two ring
conductors can be formed from the same conductor material, e.g. of
copper. The at least two ring conductors can be connected
electrically in parallel. The ring conductors can have cross
sections which are formed with a different periphery (perimeter,
circumference) and/or have different distances along the rod axis,
so that the ring conductors connected in parallel each have roughly
the same electrical resistance and/or produce an as homogeneous as
possible field distribution along the rod axis. With regard to the
mentioned cross section, it is the case of the cross section of the
conductor tube or the conductor wire of the ring conductor. A first
ring conductor and a second ring conductor for example can be
connected in parallel, wherein the first ring conductor has a
larger periphery than the second ring conductor. A distance of the
first ring conductor to the nozzle for example is larger than a
distance of the second ring conductor to the nozzle. In this case
the cross section of the first ring conductor can be enlarged
compared to the cross section of the second ring conductor.
[0051] The induction coil at least in sections can be wound in a
spiral manner and specifically in a preferably continuous manner
with a pitch which with respect to a direction parallel to the rod
axis is different to zero, wherein the windings in this section run
on the envelope of a cone symmetrical to the rod axis. The windings
or turns of the coil, in sections or continuously for example can
have an angle of more than 5 degrees, of more than 10 degrees or
more than 15 degrees to a plane which is perpendicular to the rod
axis.
[0052] A conductor forming the induction coil can be designed as a
hollow tube for leading a cooling fluid, so as to cool the
induction coil. A cross section of the hollow tube can be circular,
oval or rectangular. The hollow tube can be designed as a double
hollow tube which comprises two separate hollow chambers, for the
feed flow and return flow.
[0053] A shield which follows the contour of the rod, e.g. one
which is rotationally symmetrical, and which is open to the nozzle
can be arranged between the rod and the coil as one variant of the
heating, for melting and atomising electrically non-conductive
materials such as ceramics. The shield is preferably formed from a
material which is resistant to high temperatures and which
inductively couples, e.g. of platinum. The shield is normally
itself heated inductively and releases heat to the rod by way of
thermal radiation.
[0054] The rod itself can be designed as a crucible, as a return of
unusable residual powder, material dust and chippings. For this,
the rod can suitable have fillable cavities. Such a cavity can e.g.
be a cylindrical recess in the core, into which the residual
material can be filled.
[0055] Powder with a particularly narrow grain size distribution
can be manufactured by way of the atomisation gas being accelerated
parallel to the flow direction of the melt flow along a
comparatively short acceleration path with a length L.sub.B, at
least to the speed of sound of the atomisation gas. For example, it
can be that L.sub.B.ltoreq.5d.sub.min, wherein d.sub.min is the
already mentioned smallest diameter of the atomisation nozzle
perpendicular to the atomisation axis. The speed v of the
atomisation gas parallel to the flow direction, thus on running
through a path with a length L.sub.B is changed by an amount
.DELTA.v, wherein e.g. it is that .DELTA.v.ltoreq.0.9v.sub.0,
wherein v.sub.0 indicates the speed of sound of the atomisation
gas.
[0056] The device which is suggested here additionally to the
previous described atomisation nozzle can yet comprise a further
nozzle, which is arranged aligned to the atomisation nozzle, so
that the melt flow can also be led or is led through the further
nozzle. The further nozzle can be designed in a manner such that it
accelerates a gas which is introduced with the melt flow into the
further nozzle, in a direction parallel to the melt flow, at least
up to 0.5 times the speed of sound of the gas introduced into the
further nozzle, and specifically preferably in a laminar manner.
E.g. the further nozzle can have across section which tapers
monotonously or strictly monotonously in the flow direction of the
melt flow. The further nozzle for example can also be designed as a
Laval nozzle. The dimensions of the second nozzle can differ from
those of the atomisation nozzle. E.g. a smallest cross section of
the further nozzle can be larger than the smallest diameter of the
atomisation nozzle.
[0057] The further nozzle is preferably arranged between the
material rod and the atomisation nozzle, thus in front of the
atomisation nozzle in the flow direction of the melt flow. In
particular it can then serve for accelerating the gas introduced
into the further nozzle, even before its entry into the atomisation
nozzle. Alternatively or additionally to this, the further nozzle
can centre and/or stretch and accelerate the melt flow already
before its entry into the atomisation nozzle. However, arrangements
with which the further nozzle is arranged behind the atomisation
nozzle in the flow direction of the melt flow are also
conceivable.
[0058] Embodiment examples of the invention are represented in the
drawings and are explained in more detail by way of the subsequent
description. There are shown in:
[0059] FIG. 1 schematically, a sectioned representation of a device
according to the invention for melting a material and for atomising
the material into powder, wherein the device comprises a material
rod, an induction coil and an atomisation nozzle;
[0060] FIG. 2 schematically, an enlarged representation of the
material rod, of the induction coil and of the atomisation nozzle
from FIG. 1;
[0061] FIG. 3 schematically, a first special embodiment of the
induction coil which is shown in FIGS. 1 and 2;
[0062] FIG. 4 schematically, a second special embodiment of the
induction coil represented in the FIGS. 1 and 2;
[0063] FIG. 5 schematically, a third special embodiment of the
induction coil represented in the FIGS. 1 and 2;
[0064] FIG. 6 schematically, a fourth special embodiment of the
induction coil represented in the FIGS. 1 and 2;
[0065] FIG. 7 schematically, a plan view onto the embodiment of the
induction coil according to FIG. 6;
[0066] FIG. 8 schematically, a special embodiment for materials
which do not inductively couple, such as e.g. ceramic; and
[0067] FIG. 9 schematically, a further embodiment of the suggested
device, with which a further nozzle is arranged aligned to the
atomisation nozzle.
[0068] FIG. 1 schematically shows a sectioned representation of an
embodiment example of a device 1 according to the invention, for
the crucible-free melting a material, here titanium, and for
atomising the material into powder. The device 1 comprises a
container 2, in which a material rod 3, an induction coil 4 and an
atomisation nozzle 5 are arranged. The rod 3, the coil 4 and the
nozzle 5 are each aligned in a cylinder-symmetrical or
approximately cylinder-symmetrical manner and along a vertical axis
9. An axis of symmetry of the rod 3, an axis of symmetry of the
coil 4 and an axis of symmetry of the nozzle 5 each coincide with
the axis 9. This axis runs parallel to a z-direction 10, along
which gravity acts. An x-direction or lateral direction 11 runs
perpendicularly to the z-direction 10. The coil 4 and the nozzle 5
in particular are designed as separate components. The coil 4 is
arranged above the nozzle 5 and is distanced to this along the
z-direction.
[0069] The material rod 3 here is formed from titanium and is
partly inserted into the coil 4. A lifting/lowering device 13 is
configured to hold the rod 3 and to move it along the positive and
negative z-direction 10. The lifting/lowering device 13 can
moreover rotate the rod 3 about the rod axis with a rotation speed
of up to 200 min.sup.-1, as is indicated by the arrow 14. The coil
4 engages the rod 3 at its lower end facing the nozzle 5 and
encloses it. A cross section of the rod which is determined or
defined perpendicular to the rod axis e.g. has a rod diameter of 12
to 40 mm. The coil 4 in the region of the windings 4a and 4b has a
somewhat larger diameter than the rod 3. The coil 4 here is formed
from copper and comprises a number of windings 4a-d. The windings
4a-d at least in sections become narrower in the direction to the
nozzle 5. The first winding 4a at the end of the coil 4 which is
away from the nozzle 5 for example has a larger winding diameter
than the last winding 4d which faces the nozzle 5.
[0070] An interior of the container 2 is divided by way of a
separating or partition wall 6 into a high-pressure chamber 7
situated above the separating wall 6, and into an atomisation
chamber 8 situated below the separating wall 6, wherein the
high-pressure chamber 7 and the atomisation chamber 8 are in fluid
connection via the nozzle 5. The coil 4 and the material rod 3 are
arranged in the high-pressure chamber 7. A first gas pressure pi in
the high-pressure chamber 7, and a second gas pressure p.sub.2 in
the atomisation chamber 8 can be set via first pressure control
means 17 and second pressure control means 18. The first pressure
control means e.g. comprise a high-pressure gas accumulator with
argon, a high-pressure conduit and a high-pressure valve, via which
the argon gas can be introduced into the high-pressure chamber 7.
The second pressure control means 18 e.g. comprise a discharge air
valve and a discharge air conduit. Here, the first gas pressure
p.sub.1 is controlled to 15 bar and the second gas pressure p.sub.2
to approx. 1 bar, so that it is roughly the case
p.sub.1/p.sub.2=15.
[0071] The coil 4 is operated by an alternating current source of
approx. 100 kHz and which is not shown here, and with an electrical
power of approx 20 kW. The coil, on account of this, induces
magnetic alternating fields in the electrically conductive rod 3.
The rod 3 is inductively heated in this manner, so that it is
melted at least on the surface, at the lower rod end 15 facing the
nozzle 5. A melt flow 16 which flows downwards in the z-direction
results by way of this.
[0072] The rod end 15 of the rod 3 which faces the nozzle 5, and
the coil 4 and the nozzle 5 are shown in a slightly enlarged
representation in FIG. 2. Here and hereinafter, recurring features
are each provided with the same reference numerals. The continuous
melt flow 16 which is produced by way of the inductive heating of
the rod 3 flows downwards in the z-direction 10 and is introduced
into the nozzle through a first opening 20 of the nozzle 5 which
faces the coil 4 and the rod 4. The nozzle 5 is designed as a Laval
nozzle. The shape of the nozzle 5, in combination with the pressure
difference between the first gas pressure p.sub.1 in the
high-pressure chamber 7 and the second gas pressure p.sub.2 in the
atomisation chamber 8 effects an acceleration of the atomisation
gas in the z-reaction, emphasised here by arrows 19. Here, the
atomisation gas 19 is accelerated in the z-direction 10 and is
introduced through the first opening 20 into the nozzle 5. The
method which is suggested here can be carried out with comparably
little effort with regard to energy, since in particular, it is not
necessary to preheat the atomisation gas 19. The nozzle 5 is in
fluid connection with the high-pressure chamber 7 only via the
first opening 20. The atomisation gas 19 is thus introduced into
the nozzle 5 exclusively through the first opening 20.
[0073] The melt flow 16 is now engaged and centred by the laminar
flow of the atomisation gas 19 which is accelerated in a laminar
manner in the z-direction. The melt flow 16 then together with this
accelerating gas flow is led through the first opening 20 into the
nozzle 5 and is led through the nozzle 5. Shear stresses are
transmitted onto the melt flowing more slowly in the z-direction
10, due to the quicker atomisation gas 19. This transfer is
effected analogously to an inverse wall shear stress in the case of
laminar pipe flow, and in the flow direction causes an increase of
the pressure in the inside of the melt flow 16. In contrast, on
account of the shape of the Laval nozzle 5, a pressure drop is
effected in the flow of the atomisation gas 19 which gets quicker
and quicker. The melt flow 16 bursts and is atomised into droplets
21, as soon as the inner pressure of the melt flow 16 becomes too
large. The melt flow 16 or the droplets 31 now get through the
second opening 22 of the nozzle 5 into the atomisation chamber. The
second opening 22 is the only fluid connection between the nozzle 5
and the atomisation chamber 8.
[0074] Thus a narrowly distributed, spherically and very fine
powder of the melted material arises after the cooling and freezing
of the droplets. A titanium powder with a mass-related mean grain
diameter of 51 .mu.m is produced in the present, described
embodiment. It is then the case that d.sub.84/d.sub.50.ltoreq.2.6
for the width of the grain diameter distribution of the thus
produced titanium powder.
[0075] A parameter which is significant for the production of a
high-quality powder is the minimal nozzle cross section,
characterised by the smallest inner diameter d.sub.min (reference
numeral 23) of the atomisation nozzle 5. Here, d.sub.min=6 mm. A
plane 24 perpendicular to the nozzle axis 9 is emphasised in FIG.
2, in which plane the cross-sectional area of the nozzle is minimal
and in which the inner diameter of the nozzle 5 assumes its
smallest value d.sub.min.
[0076] It is advantageous to lead the rod 3 as closely as possible
to the nozzle 5, in order to counteract the cooling or freezing of
the melt flow 15 before the atomisation. Here, the rod 3 and the
nozzle 5 are arranged in a manner such that a smallest distance 25
between the plane 24 and the rod 3 roughly amounts to only
threefold d.sub.min, thus approx. 18 mm.
[0077] The mechanical energy which is fed or which is to be fed to
the melt flow 16 for atomisation is preferably incorporated into
the melt flow 16 by way of shear stresses of an initially resting
or essentially resting flow of the atomisation gas 19 which is not
laminarly accelerated until together with the melt flow 16. The
nozzle 5 is designed in a manner such that the flow of the
atomisation gas 10 remains laminar up to the atomisation of the
melt flow 16. The melt flow 16 is thus captured by the even slower
flow of the atomisation gas 19, accelerated, stretched along the
flow direction and tapered. The energy which is necessary for
atomisation can already be transmitted onto the melt flow 16 before
this flows through the nozzle 5.
[0078] The distance 25 between the rod 3 and the plane 24 of the
narrowest nozzle cross section and which is comparatively small
with the device described here thus moreover has the effect that
the atomisation gas is accelerated parallel to the flow direction
of the melt flow 16 along an acceleration path which is shorter
than the distance 25 between the rod and the pane 24, at least up
to the speed of sound of the atomisation gas 19. The length of the
acceleration path here therefore in particular is less than
threefold d.sub.min. The atomisation gas 19 achieves the speed of
sound when it passes the plane 24 of the narrowest cross
section.
[0079] A further effective measure, with which the cooling or
freezing of the melt flow 16 before the atomisation is prevented,
lies in leading the coil 4 as closely as possible to the nozzle, so
that the melt flow 16 before entry into the nozzle 5 where possible
still flows within the cool and is enclosed or encompassed at least
by the last winding 4d of the coil 4. In the example represented
here, the smallest distance a.sub.min (reference numeral 26)
between the end of the coil 4 which faces the nozzle 5, and the
plane 24 of the narrowest nozzle cross section, is less than double
d.sub.min, thus less than approx. 12 mm.
[0080] The rod 3, the coil 4 and the nozzle 5, as is shown in FIG.
2, are arranged in a manner such that the melt flow 16 continues to
be heated by the coil 4, in particular at least the by last winding
4d, in a region between the end of the rod 3 which faces the nozzle
5, and the nozzle 5, or between the end of the rod 3 which faces
the nozzle 5, and the plane 24 of the narrowest nozzle cross
section. For this reason, at least the last winding 4d is arranged
between the end of the rod which faces the nozzle 5, and the nozzle
5, along the flow direction of the melt flow 16. The winding
diameter of the last winding 4d here is smaller than 5d.sub.min.
The nozzle 5 is predominantly formed from a material whose specific
electrical resistance e.g. is larger than 210.sup.-2 .OMEGA.m, in
order to simultaneously prevent the nozzle 5 being heated by the
coil 4 which is led up close to the nozzle 5.
[0081] The melting of the rod 3 at its end facing the nozzle 5 is
effected in a particularly efficient manner with the arrangement
represented in FIG. 2, since the windings 4b-d in sections are each
aligned perpendicularly to the rod axis 9. The sections of adjacent
windings which are each aligned perpendicularly to the rod axis 9
are connected by way of oblique sections which each bridge a
constant pitch G.
[0082] It is important for a melt rate (mass per time) at which the
rod 3 is melted, to be large enough to produce a continuous melt
flow 16, so that the inductive heating of the jet can be effected
in a particularly effective manner. The melt rate should e.g. be at
least 0.5 kg per minute or at least 1 kg per minute. The melt rate
which is necessary for producing a continuous melt flow 16 is of
course dependent on the special characteristics of the melted
material and can vary from material to material (e.g. viscosity,
surface tension)
[0083] Schematic special embodiments of the coil 4 are shows in
FIGS. 3 to 7.
[0084] FIG. 3 shows an embodiment of the coil 4, with which the
windings 4bd are spirally wound and run on the envelope 26 of an
imagined cone, said cone being symmetrical with respect to the rod
axis 9. With a complete revolution by 360 degrees, each winding
thereby overcomes pitch G. A diameter of a conductor or conduit
tube 27 which forms the coil 4 is indicated at P. G is normally
somewhat larger than P. It can be for example that G.gtoreq.1.5P.
The conductor 27 is designed as a hollow tube of copper for cooling
by way of cooling fluid. The outer diameter P of this hollow tube
can e.g. be 12 mm. A wall thickness of the tube can be 2 mm.
[0085] The embodiment example of the coil 4 according to FIG. 4
differs from that according to FIG. 3 in that the coil here
comprises two conduit tubes 27a and 27b which are connected
electrically in parallel and which again are each wound in a spiral
manner and become conically narrower towards the lower end. The
hollow tubes 27a and 27b are likewise formed from copper. Their
outer diameter P here however should only be 6 mm. The wall
thickness is only 1 mm. The conduit tubes 27a and 27b can thereby
be wound in a significantly simpler manner than the conduit tube 27
according to FIG. 3 The pitch G of the conductors 27a and 27b here
is also e.g. 18 mm in each case, but the lowermost diameter d.sub.u
is significantly smaller.
[0086] With the embodiment of the coil 4 which is shown in FIG. 5,
this comprises a conductor 28 which is designed as a double hollow
tube with a rectangular cross section. A height of the cross
section of the conductor 28 is indicted at H, and a width at B. The
double hollow tube comprises two individual hollow tubes 28a and
28b which are joined to one another and whose cavities are
separated, are thus not in fluid connection. The hollow tubes 28a
and 28b each have a square cross section with a side length
H.sub.i, wherein H=2H.sub.i.
[0087] FIG. 6 shows a further embodiment of the coil 4, with which
the windings 4b-d are each designed as sleeves shaped roughly in
the manner of a horseshoe, of which each is aligned perpendicularly
to the coil axis 9. The coil 4 according to FIG. 9 thus produced
fields with a particularly high symmetry with regard to the coil
axis 9. The material rod 3 can thus be melted in a particularly
uniform manner.
[0088] Each of these sleeves forms a ring conductor which is
electrically interrupted at one location, and is almost closed,
thus encloses the axis 9 in each case by up to 340 degrees. The
electrical interruptions are designed as air gaps 31b-d.
[0089] The windings 4a-d are connected electrically in parallel and
are designed each as a hollow tube for leading a cooling fluid. The
hollow tubes forming the windings 4a-d are each composed of two
complementary pieces which are L-shaped in cross section. A cross
section of each hollow tube therefore has the shape of a
parallelogram. The hollow tube 29 forming the winding 4b is
composed for example of the pieces 29a and 29b, and these are
connected by way of solder connections 30. The outer and inner
surfaces of the pieces 29a and 29b in turn form cone sections. The
inner diameters d.sub.1, d.sub.2 and d.sub.3 reduce in the
z-direction 10. Distances t.sub.1 and t.sub.2 between the windings
and which are determined which is to say defined along the
z-direction 10 are equally large.
[0090] The windings 4b-d are each manufactured of the same conduit
material and each have a different periphery. The flows flowing in
the individual windings 4b-d connected in parallel can be adapted
by way of giving the windings 4b-d different cross sections in each
case, for an as uniform as possible melting of the rod at its lower
end. Here, it is shown that the heights H.sub.1, H.sub.2 and
H.sub.3 of the windings 4b-d are different in each case. In
particular, the heights H.sub.1, H.sub.2 and H.sub.3 e.g. increase
linearly from the bottom to the top with the diameters d.sub.1,
d.sub.2 and d.sub.3. Due to this, one succeeds in approximately
equal flows flowing in each case in the windings which are
connected in parallel, so that the rod 3 is melted at its surface
at the lower end in an as uniform as possible manner.
[0091] The air gaps 31b-d of the different windings 4b-d are
additionally rotated to one another by angles .alpha..sub.1 and
.alpha..sub.2, as can be deduced from FIG. 7.
[0092] FIG. 8 shows an embodiment of the device 1, with which in
particular non-inductively coupling materials can be melted, e.g.
ceramic. A shield 32 which follows a contour of the rod i.e. is
rotationally symmetrical and which is open to the nozzle, is
arranged for heating the rod 3 by way of the coil 4. The shield 32
is formed from a material which is high-temperature resistant and
which couples in an inductive manner, e.g. of platinum. The shield
32 is itself normally inductively heated and releases heat to the
rod by way of thermal radiation.
Trail Results:
[0093] With a first trial with a rod of aluminium with a diameter
of d=50 mm and which is to be melted, a power of about 14 kW was
transmitted by a high-frequency transformer, at a speed of the rod
of about 40 min.sup.-1, and the magnetic field of this transformer
was excited at about 105 kHz after coupling in. A dripping and not
yet continuous melt flow at a nozzle atomisation pressure (gas
pressure in the high-pressure chamber) of 10 bar was produced in
the case of a nozzle of a very good conductor, wherein the position
of the melting region was not easily to recognise due to the low
melt temperature of aluminium.
[0094] 16 kW was transmitted at 101 kHz in the case of another
trial with a 50 mm rod of stainless steel 1.4462. Again, the speed
was about 40 min.sup.-1 and the nozzle was of a good conductor. A
continuous melt flow could be produced for a short time at a first
gas pressure of 10 bar, otherwise only a dripping material
discharge.
[0095] Very different powers in the region of approx. 25-35 kW at
107 kHz were transmitted in the case of a further trial with a 38
mm rod of stainless steel 1.4462. The speed was again about 40
min.sup.-1 and the nozzle this time was of a non-conductor, so that
a particularly small distance could be set between the coil and the
Laval nozzle. Moreover, the mentioned two-stage nozzle was applied
here. A continuous melt flow could be produced at a nozzle
atomisation pressure of 20 bar. The mean grain size hereby was
d.sub.50=49 .mu.m and d.sub.84/d.sub.50 was equal to 2.68.
[0096] A power of about 35 kW was transferred at a frequency of 112
kHz on nozzle atomisation of titanium rods with 20 mm diameter at a
nozzle atomisation pressure of 17-19 bar with respect to
atmosphere, with a Laval nozzle of a non-conductor with a two-stage
contour. The speed was the same as above. A mean grain size of
d.sub.50=51.4 .mu.m at d.sub.84/d.sub.50=2.60 and in a part-flow of
23.7 .mu.m at d.sub.84/d.sub.50=1.78 resulted.
[0097] FIG. 9 shows a modified embodiment of the device 1 of FIG.
1. Features which were already previously described and in
particular in the context of FIG. 1 continue to be indicated with
the same reference numerals. The device 1 according to FIG. 9
differs from the device 1 according to FIG. 1 in that a further
separating wall 34 is arranged along the z-direction 10 between the
separating wall 6 and the material rod 3. A through-opening in the
further separating wall 34 forms a further nozzle 33. A cross
section of the further nozzle 33 tapers in the positive z-direction
10, and thus in the flow direction of the melt flow 16, in the
shape of a cone. A nozzle axis of the further nozzle 33 coincides
with the axis 9, so that the atomisation nozzle 5 and the further
nozzle 22 are arranged in an aligned manner.
[0098] The melt flow arising at the rod end 15 is this firstly
introduced into the further nozzle 33. This is designed in a manner
such that it accelerates the atomisation gas 19 entering the
further nozzle 33 at the entry opening of this which faces the
material rod 3, to at least to 0.5 times the speed of sound of the
atomisation gas 19, parallel to the flow direction of the melt flow
16, thus along the positive z-direction 10. The melt flow 16 is
thus centred and stretched already before the entry into the
atomisation nozzle 5. It has been found that this can improve the
quality of the powder produced in the atomisation nozzle 5 to an
even greater extent, with regard to the achieved grain size as well
as with regard to the achieved grain size distribution width. An
area of the smallest cross section of the further nozzle 33 which
is determined perpendicularly to the nozzle axis 9 in FIG. 9 is at
least fivefold the area of the smallest cross section of the
atomisation nozzle 5. However designs of the further nozzle 33
which differ from this are also conceivable
[0099] A suitable pressure difference is required on both sides of
the separating wall 34, so that the further nozzle 33
(pre)accelerates the atomisation gas 19 as described previously.
This pressure difference is produced by the previously described
first pressure control means 17 and by third pressure control means
35. The third pressure control means as is the case with the first
pressure control mans 17 comprise a high-pressure conduit and a
pressure control valve, which are connected to a high-pressure gas
accumulator with argon and via which argon gas can be introduced
into an intermediate space 36 between the separating walls 6 and
34. The pressure control means 17, 18 and 34 for example can be set
in a manner such that the gas pressure p.sub.3 in the intermediate
space 36 is about p.sub.3=(p.sub.1+p.sub.2)/2, wherein p.sub.1 and
p.sub.2 as described previously indicate the gas pressure in the
high-pressure chamber 7 and in the atomisation chamber 8. The
pressure control means 17, 18, 35 in this case are to be set such
that p.sub.2<p.sub.3<p.sub.1.
* * * * *